Patent Application
20120110977
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The present invention relates in general to ion thrusters for space propulsion.
Ion thrusters or engines have played vital role in space propulsion for wide ranges of applications, such as low thrust precision attitude control, orbit transfer and interplanetary flights. One of crucial parameters of ion thrusters, which determine its applicability to specific missions, is the thrust density, the ratio of the thrust to the area of the exit nozzle/electrode. The high thrust density correlates with a smaller accelerator grid area that is essential in minimizing the construction, operation and lifting costs of the ion thrusters. Although the power to thrust conversion efficiency and Isp of ion thrusters can be much higher than conventional chemical thrusters, the ion thrusters currently are not used for missions requiring large thrusts in the order of multi megawatts, because their construction and lifting costs are prohibitive. Therefore, the methods of increasing thrust density of ion thrusters can significantly broaden their application scopes have been extensively sought for.
The operation of the ion thruster relies on acceleration of ions. Thus, the space-charge limitation of the ion acceleration process limits the thrust density. Currently, most of ion thrusters use atomic species, such as Xe or Hg making the ion thruster practical for only a limited range of missions. Extensive research efforts have attempted to increase thrust density to levels that would lead to an attractive ion thruster with wider applicability with the use of heavier ion species than Xe or Hg without success. The method of increasing thruster density can be guided by a physical theory by Child-Longmuir law, and according to this law, the thrust density, Ta, of an ion thruster can be given by:
Ta∝mi2Isp4, (1)
where Isp is the specific impulse and mi is the ion mass of the propellant. For a specific mission with a fixed Isp, the higher the ion mass is, the higher is the thruster density. Because the thrust density is proportional to square of the ion mass, even small change in ion mass can increase the thrust density significantly. For example, the atomic mass of the most popular propellant Xe is 131, and any fuels with atomic or molecular mass greater than 131 would increase the thrust density over the current limit.
Molecular or cluster ions can potentially increase ion mass significantly, however, with highly increased probability of fragmentation, which negates the effect of increased ion mass on thrust density. Therefore, the usage of molecular or cluster ions for ion thrusters has not been successful until now. Fullerene clusters, such as C60, have much larger masses than Xe, yet under favorable thermodynamic conditions, they behave like atoms in terms of resisting fragmentation. In addition to their larger mass than that atomic species, fullerene clusters have lower ionization potentials, thus require lesser energy for ionization than atomic species. Fullerene clusters can be sublimated at relatively low temperatures without fragmentation, and their vapors behave like atomic vapors. Therefore, the usage of C60 for propellant for ion thrusters has been extensively investigated by researchers over two decades.
For example, C60 clusters, cardinal clusters among fullerenes, have a mass of 720. If thrust operation conditions are kept the same, the thrust density of C60 ions would be greater than Xe ions by a factor of (720/131)2˜30 according to Eq. 1. For example, a high thrust mission with a thruster beam power of 10 MW and Isp-5,000 with Xe as propellant would need a grid area of 18 m2, which is too large for economically viable construction and lift into space. If a similar ion thruster can be operated with C60 fuel, the required grid area decreases to 0.60 m2, which is economically viable for a wide range of space missions. The heavier fullerenes, such as C72 or C84 would have better size-reduction effects. The chemistry of fullerenes has recently produced extensive classes of fullerene derivatives, fullerene nanotubes, and fullerene nanotube derivatives. Successful usages of these large stable clusters will further increase the thruster density. Therefore, fullerene-family fuels may open new doors for electrostatic propulsion, if they can be successfully used in ion thrusters.
Extensive research and development efforts for fullerene ion thrusters have at best produced engines with undesirably low fuel usage efficiency due to serious propellant deposition and other problems resulting from premature fragmentation before full electrostatic acceleration. Previous state-of-the-art fullerene ion thrusters have used traditional ionization methods including DC and RF discharge plasmas. An example C60-based ion thruster system is described in U.S. Pat. No. 5,239,820, entitled “Electric Propulsion Using C60 Molecules,” issued Aug. 31, 1993, to Leifer et al., the disclosure of which is incorporated herein by reference. The prior usage of such ion thruster structures and operation methods with cathodes and hot filaments in DC or RF discharge chambers has not successful in realizing efficient and practical fullerene ion thrusters. A number of publications similar to the above mentioned C60 ion thruster system reported a failure of obtaining sufficiently high efficiency of fullerenes for rendering C60 ion thruster practically and economically viable, which have been incorporated herein by reference. Other methods include the usage of charge exchange of fullerene with rare gas ions generated in discharge chambers in a modified configuration of Hall thrusters. An example such ion thruster system is described in Hruby, et al., “A High Thrust Density, C60 Cluster Ion, Thruster,” AFOSR Final Report No. 49620-94-C-0006, September 1996, the publication of which is incorporated herein by reference. Such approach also resulted in similar inefficiency of fullerene usage to the above mentioned references.
None of the existing approaches so far resulted in a practical fullerene ion thruster, mainly because their ionization methods for generating fullerene cluster ions induce extensive fragmentation of fullerene clusters resulting in very low efficiency fullerene usage. Therefore, other innovative ion thruster structures and operation methods have been sought for. The present invention solves these problems in existing fullerene ion thrusters with the use of VUV photoionization followed by thermal effusion of fullerene clusters, which has negligible fragmentation during ionization process, thus promises cost effective and practical fullerene ion thrusters for a wide range of space propulsion applications.
The inventor realized that the fundamental problem in the existing fullerene ion thrusters is in the ionization process of fullerenes and transportation process of the fullerene ions ant that the problem can be avoided by using a much gentler ionization method than the ones used in previous works or inventions. The present state-of-the-art fullerene ion thrusters use either electron impact ionization or charge exchange with other rare gases in hot filament environment as used in existing methodologies. Such ionization processes can deposit large internal energy into fullerenes after ionization. The hot filament discharge environment can also rapidly destroy fullerenes. These hot fullerene ions can readily fragment in very short time during acceleration even without collision with rare gas atoms. To make the situation worse, in the traditional fullerene thrusters, transportation of fullerene ions is performed in the mixture of fullerene and rare gas atom vapors at relatively high pressure resulting in further extensive collisional fragmentation.
The salient feature of the present invention lies in the usage of VUV photoionization of molecular beams of fullerene clusters generated by molecular beam sources, including but not limited to Knudsen cells. The photoionization with controlled photon energy can softly ionize fullerenes without depositing extensive internal energy that can fragment fullerene ions. The photoionization cross sections above the ionization potential of fullerene are well investigated. For example, the ionization potential of C60 is 7.58 eV, and its photoionization cross section at 10 eV is ˜10−16 cm2, which is sufficient for efficient and soft photoionization with minimal fragmentation. The reason for this is that during photoionization at photon energy close to the photoionization threshold, the thermal energy, which induces fragmentation, imparted to fullerenes is minimal, and most of photon energy is used for expelling electrons.
The present invention can also minimize fragmentation during evaporation of fullerenes, because the present invention does not require enclosed structures for ionization process, which are required for containing DC or RF energies in prior arts. Since the present invention does not require such heavy enclosed structures for ionization, in principle, the ionization area can be arbitrarily large without increasing the overall weight of the thruster. This advantage allows to lower evaporation temperature of fullerenes sufficiently below the fragmentation threshold temperature resulting in minimal thermal fragmentation. For example, if the photoionization region has a diameter of 30 cm, the unit ionization efficiency can be achieved with fullerene densities in the order of 3×1014/cm3. Such a fullerene number density can be achieved by heating the fullerene solid to 650 C well below the thermal fragmentation temperature of 750 C by molecular beam sources including but not limited to Knudsen cells. With such configuration, almost all fullerenes can be efficiently ionized and accelerated together, thus the collisional fragmentation can be minimized as well. These advantages can not be found in prior art.
The photoionization of fullerenes require intense VUV photon sources with photon energies in excess of 10 eV, well above the photoionization threshold energy, 7.58 eV, of fullerenes. The ideal VUV photons should have high enough photon energy to have reasonably large photoionization cross sections, but low enough photon energy not to fragment fullerenes. The ideal photon energy thus is ˜10-20 eV. Such photon energy can be readily achieved by the above mentioned VUV photon source technologies. The new development in VUV photon lamps, including but not limited to, rare gas resonance lamps, rare gas excimer lamps, now provides the required high flux of VUV photons with high efficiency. Furthermore, the scaling up of such VUV photon source seems straightforward. For example, a 10 MW ion thruster for interplanetary mission with 10 kV (Isp˜5,000 sec) acceleration would require a cluster ion beam of 1 kA with a ion flux of 6.3×1021 ions/sec. This would require at least 6.3×1021 VUV photons per second. With a photon energy of 10 eV, the required photon source power is 10 kW, which is well within reach of near-future VUV source technologies.
Another advantage of the present invention is that it does not use rare gas for ion transportation in addition to fullerene fuel. Therefore, the structure of the present invention can be considerably simpler and lighter than the exiting fullerene ion thrusters that use conventional discharge plasma technologies. Furthermore, the thrust efficiency of the present invention is significantly high because does not require mixing with rare gas, which reduces the overall thrust efficiency of fullerene-based ion thrusters. Qualitatively, the summary of the advantages of the present invention over the conventional exiting ionization method for fullerene ioni thrusters is presented in Table 1.
Another important aspect of the present invention is in its ability of ionizing with minimal fragmentation of functionalized fullerenes, fullerene nanotubes, and fullerene nanotube derivatives, which have larger mass than fullerenes and can be tailored for mission specificities. Currently, chemists in the world have successfully produced bulk quantities of varieties of functionalized fullerenes (fullerene derivatives), such as C60-F48, which was recently shown to be evaporable without fragmentation. For example, C60-F48, has a mass of is 1632.
The thrust density of C60-F48 can be 5 times higher than that of C60 alone, and 150 times higher than that of Xe. The potential usage of other heavier functionalized fullerene, fullerene derivatives, fullerene nanotubes, and fullerene nanotube derivatives can further increase the thruster density. Therefore, the successful usage of such fullerene derivatives in the present invention will result in more compact and lighter ion thrusters, thus can greatly expand the usage of ion thrusters for unprecedented space mission applications further beyond ion thrusters using fullerene ions. The ionization with minimal fragmentation of such large fullerene derivatives can be readily achieved by the present invention that uses VUV photoionization.
Elements in the figures have not necessarily been drawn to scale in order to enhance their clarity and improve understanding of these various elements and embodiments of the invention. Furthermore, elements that are known to be common and well understood to those in the industry are not depicted in order to provide a clear view of the various embodiments of the invention.
In the following discussion that addresses a number of embodiments and applications of the present invention, reference is made to the accompanying drawings that form a part hereof, in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and changes may be made without departing from the scope of the present invention.
The fundamental principle of the present invention lies in the usage of photoionization for ionizing fullerene-family clusters, including but not limited to fullerene clusters, fullerene derivatives, nanotubes, and nanotube derivatives, generated by a molecular beam source, including but not limited to various effusion sources, such as Knudsen cells. In the following descriptions, fullerene clusters can represent fullerene-family clusters or molecules without departing from the scope of the present invention.
The number of electrodes can be varied depending on applications and preferred thruster configuration. The voltages between electrodes can be varied depending on applications and preferred thruster configuration. The electrodes can be solid plates, apertures or grids, or their combinations, depending on applications and preferred thruster configuration. Other components that are not shown in
The number of thermal effusion sources can be greater than one without departing from the scope of the present invention. The number of electrodes can be varied depending on applications and preferred thruster configuration. The voltages between electrodes can be varied depending on applications and preferred thruster configuration. The electrodes can be solid plates, apertures or grids, or their combinations, depending on applications and preferred thruster configuration. Other components that are not shown in
The number of thermal effusion sources can be greater than one without departing from the scope of the present invention. The number of electrodes can vary depending on applications and preferred thruster configuration. The voltages between electrodes can vary depending on applications and preferred thruster configuration. The electrodes can be solid plates, apertures or grids, or their combinations, depending on applications and preferred thruster configuration. The VUV sources can be of a point, planar, slit, annular or combination source configuration. Other components that are not shown in
The number of thermal effusion sources can be greater than one without departing from the scope of the present invention. The number of apertures on the thermal effusion source can vary depending on applications and preferred thruster configuration. The number of VUV photon sources can vary depending on applications and preferred thruster configuration. In some situations, the VUV sources can be arranged in a circular fashion with the VUV photon beams directed to the center of the fullerene cluster beams. The number of electrodes can be varied depending on applications and preferred thruster configuration. The voltages between electrodes can be varied depending on applications and preferred thruster configuration. The electrodes can be solid plates, apertures or grids, or their combinations, depending on applications and preferred thruster configuration. The VUV sources can be of point, planar, slit or annular source configuration. Other components that are not shown in
More specifically, the fullerene clusters used in the present invention can be replaced with fullerene derivatives or functionalized fullerenes, including but not limited to fluorinated, hydrogenated, hydroxylated, chlorinated, and brominated fullerenes without departing from the scope of the present invention. The examples of fluorinated fullerene derivatives include but not limited to C60F36, C60F48, and C60F60. The examples of hydroxylated fullerene derivatives include but not limited to C60(OH)n with n can be 1-60. The examples of hydrogenated fullerene derivatives include but not limited to C60Hn with n can be 1-60. The examples of chlorinated fullerene derivatives include but not limited to C60Cln with n can be 1-60. The fullerene derivatives may have attachment of other organic and inorganic molecules without departing from the scope of the present invention.
The fullerene clusters used in the present invention can be replaced with fullerene nanotubes or their functionalized forms, including but not limited to fluorinated, hydrogenated, hydroxylated, chlorinated, and brominated fullerene nanotubes without departing from the scope of the present invention. The fullerene nanotube derivatives may have attachment of other organic and inorganic molecules without departing from the scope of the present invention.
The present application claims priority, under 35 USC §199(e) and under 35 USC §120, to the U.S. Provisional Patent with Application Ser. No. 61/409,963 filed on Nov. 4, 2010, the disclosure of which is incorporated herein by reference in its entirety.
This invention was made with Government support under Contract No. HDTRA1-10-C-0088 awarded by the Defense Threat Reduction Agency. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61409963 | Nov 2010 | US |
